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WEDNESDAY, May 30, 2012 (MedPage Today) — Women who regularly worked the night shift were more likely to develop breast cancer, a Danish military study affirmed.

Any night work tended to boost the adjusted odds of breast cancer 40 percent compared with never working nights, reported Johnni Hansen, PhD, and Christina F. Lassen, MD, both of the Danish Cancer Society in Copenhagen.

But that risk rose with more years of night shift work and a higher cumulative number of night shifts worked, more than doubling in risk for women who had the night shift most weeknights for at least 6 years.

The risk was particularly great for "morning people" who had to work intense night shifts, they reported online in Occupational and Environmental Medicine.

Night work seems to be on the rise worldwide, with 10 percent to 20 percent of the workforce in Western countries now working schedules that start after 5 p.m. and end before 9 a.m., the group noted.

Most, though not all, prior case-control and large prospective studies have also suggested a risk for breast cancer from such shift work.

"Night work can disrupt circadian rhythms, suppress production of the pineal hormone melatonin and result in sleep deprivation, all of which affect hundreds of metabolic and physiological processes, including hormone production, cell cycling and apoptosis, which in turn may increase the initiation, progression and growth of human tumors, including breast cancer," they wrote.

As part of a larger study of occupational and non-occupational exposures and risks for cancer in the Danish military, Lassen's group compared 218 cases of breast cancer to 899 matched controls within a nationwide cohort of 18,551 female military employees.

Night shift work, defined as lasting for at least 1 year but not including overtime, didn't have a significant impact if women worked it only once or twice a week.

"This is consistent with the observation that one or two night shifts will not change the timing of melatonin production and thereby not initiate circadian disruption," Hansen and Lassen noted.

But longer duration — night shift work at least three times a week — boosted risk.

The odds were 2.1 times higher for 6 to less than 15 years on that schedule and 2.5 times greater with 15 or more years of at least three nights a week, both statistically significant adjusted risks compared with never being on the night shift.

The risk rose with greater cumulative exposure too.

Self-selection bias has been suggested as a possible explanation for these associations if "night owls" are more likely to accept or stay on the night shift and are more genetically susceptible to breast cancer than "larks", women who prefer mornings.

However, the study showed greater risk actually among the larks than among the night owls.

Inadequate vitamin D from missing out on sun exposure by working nights has also been proposed as a mechanism.

Again, though, the study did not favor that explanation, finding that the night workers were more likely to report sunbathing.

Further exploration is warranted in larger studies, the researchers argued.

They cautioned that this retrospective study may have suffered from potential recall bias. Another limitation was the modest rate of participation of military members in the questionnaire the study was based on.

Also, women who had died from breast cancer were not included, which may have underestimated the associations.